“Secular trends (2001–2020) in physical fitness as a health component in physiotherapy students from Bydgoszcz (Poland)”

The aim of this study was to assess changes in the components of physical fitness that are conducive to the health of young people surveyed between 2001 and 2020. Hypotheses were formulated about an increase in the thickness of skinfolds, especially among women, the deterioration of the results of fitness tests and the lack of conditioning of the examined changes with socioeconomic factors. Every year, physiotherapy students at the Collegium Medicum in Bydgoszcz, Nicolaus Copernicus University in Toruń (Poland) were examined for body height and mass, skinfold thickness, flexibility, static strength, abdominal muscle strength, functional strength and endurance. The results were obtained from 1161 female students and 464 male students. Similar trends were observed for most of the studied characteristics in people of both sexes. In male students, secular trends towards a decrease in the thickness of biceps skinfolds (R2 = 0.455, p = 0.002) and lower leg skinfolds (R2 = 0.314, p = 0.015) were found. In female students, secular trends towards an increase in the body mass (R2 = 0.41, p = 0.003), a decrease in the thickness of skinfolds over the biceps (R2 = 0.477, p = 0.001) and decreased flexibility (R2 = 0.283, p = 0.023) were found. Male sex and the thickness of suprailiac skinfolds were frequent predictors of the, assessed motor abilities and socioeconomic factors did not significantly affect their maintenance. The obtained results, except for a few secular trend examples of the somatic features (male student’s age and calf skinfold, female student’s body mass, male and female student’s biceps skinfold) and flexibility in women, showed their stability and the lack of conditioning by social factors and by the fitness test. Attention to the appropriate level of the observed components is justified, especially in light of the identified trend that showed their deterioration.


Materials and methods
During the years 2001-2020, cross-sectional studies were carried out on physiotherapy students from the Collegium Medicum in Bydgoszcz, which has been part of the Nicolaus Copernicus University in Toruń since 2004.The Ludwika Rydygier Medical Academy, which was the original name of the university, was the first of the Polish medical academies to offer education in the field of physiotherapy.Bydgoszcz is a large provincial city located in the west-central part of Poland, with more than 350,000 population and with academic traditions.The research was carried out during classes; it was initially conducted in the third year of long-term master's studies and later in the first and second years of bachelor's studies.The change in the study model did not allow the research to be carried out in 2014, and beginning in 2019, the research included male and female master's students.Therefore, the location of the subject "movement education and movement teaching methodology" in the study program determined the stage of the study at which the research was carried out and, thus, also determined the age of the students.Until 2010, during recruitment to studies, a fitness test was carried out, which included measuring the static strength, endurance and agility, as well as assessing swimming skills.With the exception of the last group of male and female students, whose research period coincided with the pandemic and lockdown, all surveyed students took part in physical education classes (60 h), movement education with movement teaching methodology (110 h) and summer and winter camps (110 h).

Test procedures
At the end of each year of the observation, the body height and mass were measured.These results and socioeconomic characteristics, as well as the basic information about the study process presented above, were used in the article on the components of physical fitness conducive to achieving motor achievements 19 .The division of the material into two studies was dictated by indications for a new, instrumental (heterotelic) interpretation of the results of physical fitness testing proposed by a large group of EUROFIT authors, as well as Polish authorities on human motor skills research 20,23 .The decision to present the material in two separate works was also influenced by its considerable volume, which, if contained in one article, could make it illegible or require selective presentation.Hence the similarity in the description of the basic somatic features (height and body weight) and socioeconomic conditions of the studied groups.The majority of the surveyed male and female students were residents of large cities (65.97%), a large proportion of whom were from Bydgoszcz (20.25%), and almost all of them were graduates of general secondary schools (95.75%).The parents of some surveyed students received secondary education (fathers, 35.82%; mothers, 42.03%), and a large percentage, especially among mothers, completed higher education (42.58%).
Basic somatic characteristics were supplemented with thickness measurement of triceps, biceps, subscapular, abdominal, suprailiac and calf skinfolds 24 .Measurements of the body height and the thickness of the skinfolds were made using an anthropometer, with an accuracy of 1 mm, and a calliper, with an accuracy of 0.2 mm, from the Swiss company GPM.The body mass was determined using an electronic scale manufactured by DB-1H Castex in China, with an accuracy of 0.01 kg.The Eurofit test was used to examine the health-promoting components of physical fitness in the following areas: flexibility (sit and reach [SAR]), static strength of the right and left hands (handgrip [HGR]), trunk strength (sit-ups [SUP]) and cardiorespiratory endurance (endurance shuttle run [ESR]).Considering the structure of motor skills, which are a component of physical fitness or are used to describe it, the tests conducted reflected the level of fitness (energy) abilities and only indirectly reflected information (coordination) abilities 5,20,25 .The Eurofit test is a randomized, widely used tool based on simple motor tests that do not require special motor skills.The commencement of the motor tests was preceded by the

Ethics
The research was carried out with the consent of the Bioethics Committee at the Collegium Medicum im.Ludwika Rydygiera of Nicolaus Copernicus University in Bydgoszcz (No. KB 44/2004-extended every 4 years until 2020).All men and women were informed about the purpose of the study, the type and duration of the effort, and the possibility of withdrawing from the study without giving any reason.After this information was provided, the students provided informed consent to participate in the study.All procedures contributing to this work followed the ethical standards of the relevant national and institutional human experimentation committees and the 1975 Declaration of Helsinki, as amended in 2008.

Statistics
The results of the measurements are presented in tables containing average values and standard deviations.The relationships of the changes in the average annual values of the examined characteristics between women and men were assessed with Pearson's linear correlation coefficients.The values of the coefficients were interpreted as the strength of the linear relationship between the characteristics.Formally, it is the normalized covariance of two defined variables.Changes in the measured values over time were assessed using simple linear regression analysis to determine the regression coefficients so that the model could predict the value of the dependent variable.Analysis of the impact of features on fitness results was carried out using the multiple linear regression method, which is used to fit linear or linearized models between one dependent variable and more than one independent variable.
The following characteristics were used as potential predictors: body height and mass, thickness of skinfolds, fitness exam in the recruitment procedure for studies, sex, age, place of residence, type of completed school, and father's and mother's level of education.

Results
Table 1 summarizes the sizes of the studied groups of students and the values of basic somatic features 19 .The descriptive statistics indicate that in all years of observation, women accounted for significantly larger shares of the sample, with slightly lower average ages and standard deviations.The men surveyed in 2020 were characterized by a greater average body mass than that in earlier years.

Results of measuring somatic features
Table 2 presents the results of the skinfold thickness measurements for the groups of male students, and Table 3 presents the results for the groups of female students.
As the data show, the examined age groups of men were characterized by lower average skinfold measurements and, with the exception of the skinfolds measured on the abdomen and on the suprailiac area, by smaller standard deviations, which confirms that the dispersion of individual measurements of somatic features in men was smaller than that in women.

Motor skill test results
Table 4 presents the results of the fitness tests for the male students, and Table 5 presents the results for the female students.
The statistical data presented in the tables show that, except for flexibility, the surveyed age groups of men were characterized by higher average results of the fitness tests, and with the exception of the trunk strength, by larger standard deviations, which indicate a greater dispersion of individual results of fitness tests in men than in women.Those surveyed in 2020, especially women, were characterized by a lower average trunk strength than the student groups in earlier years.

Data analysis
The relationships of the changes in the average annual values of the examined characteristics between women and men were assessed with Pearson's linear correlation coefficients, and the results are presented in Table 6.
With the exception of Pearson's linear correlation coefficients calculated for the body mass, skinfold under the scapula and functional strength, all others were statistically significant, indicating that the vast majority of changes in the characteristics under observation were similar between men and women.
The dependence of the studied features on time was assessed using a simple linear regression model, with the coefficients for men presented in Table 7 and those for female students presented in Table 8.Significant secular trends are graphically illustrated in Figs. 1, 2, 3, 4 and 5.
As the data for the age of men show, the model is significant and indicates a decrease of 0.068 years in the average age of students, along with the progressive change of the surveyed age groups.Regarding the age of women, the model is not significant and indicates that the values of the trait are maintained at similar levels.www.nature.com/scientificreports/For the body height of men and women, the models are not significant and indicate that the values of the feature are maintained at similar levels.For men's body mass, the model is not significant, but for that of women, the model is significant and indicates a year-on-year increase of 0.145 kg in the average value of this feature 14 .
For the thickness of the skinfold at the front of the arm in men, the model is significant and shows a year-onyear decrease of 0.412 mm in the value of this feature.For women, the model is also significant and indicates a year-on-year decrease of 0.37 mm in the value of this feature.
For the remaining skinfold thicknesses of men and women, with the exception of that above the calf in men, the models are not significant and indicate that the values of these features remain at similar levels.For men's calf skinfolds, the model is significant and shows a year-on-year decrease of 0.22 mm in this feature.
For motor tests, with the exception of women's flexibility, the models are not significant and indicate that the values of these features remain at similar levels.For men's body flexibility, the model is not significant, while for  www.nature.com/scientificreports/ that of women, the model is significant and indicates a year-to-year decrease of 0.179 cm in the average value of this feature.
Multiple linear regression was also used to examine which of the somatic and sociometric characteristics adopted for the analysis significantly affected the results of the fitness tests, provided that all other indicators were the same.The most important features were selected using the sequential feature selection "forwards" method, maximizing the R-square index.Its value and the significance level of the regression models ≤ 0.001 determined the predictors, as shown in Table 9.
For body flexibility, R-square = 0.027, p = 8e-11, and the model explained 2.7% of the variability, with a significance of 0.001 for the whole model.Increasing the body height by 1 cm reduced the flexibility score by − 0.144 cm.For the static force of the right hand, R-squared = 0.631, p = 0.0, and the model explained 63.1% of the variability, with a significance of 0.001 for the whole model.Increasing the body mass by 1 kg increased the static force by 0.2196 kG, and male sex increased the static force by 17.269 kG.
For the static force of the left hand, R-square = 0.627, p = 0.0, and the model explained 62.7% of the variability, with a significance of 0.001 for the whole model.Increasing the body mass by 1 kg increased the static force by 0.3236 kG.Increasing the value of the scapular fold by 1 mm caused a decrease in the static force of − 0.2977 kG, and male sex increased the static force by 13.8856 kG.
For the abdominal muscle strength, R-square = 0.214, p = 3.35e-81, and the model explained 21.4% of the variability, with a significance of 0.001 for the whole model.Increasing the value of the suprailiac skinfold by 1 mm caused a decrease in the abdominal muscle strength of − 0.1154 points, and male sex increased the abdominal muscle strength by 3.5319 points.
For the functional strength, R-square = 0.424, p = 2.89e-183, and the model explained 42.4% of the variability, with a significance of 0.001 for the whole model.Increasing the body mass by 1 kg caused a decrease in

Discussion
In the present study, a slightly different picture of the secular trend changes in the components of physical fitness supporting health in young adults was obtained compared with that based on the material collected in the analysis of the components supporting physical fitness 18 .Thus, the lack of a clear relationship between the energy factor and health and the need to separate the autotelic and heterotelic interpretations of the results of fitness tests were indirectly confirmed 20,23,26 .The results, except for body flexibility, were more favourable in the male group, which is consistent with the literature data 27,28 .This may suggest that men, who maintain better health than women, will be able to meet therapeutic standards for a longer time, which may cause numerous overloads and injuries in this profession 29,30 .
The assessment of the relationships of changes in the average annual values of the examined characteristics between women and men, with the exception of three characteristics, showed similar trends, which indicates similar reactions of the organisms of men and women to exogenous factors occurring during the course of studies.These and the previously presented research results are consistent with the observation of physical fitness components supporting motor achievements 19 .Thus, the lack of similarity in the changes in the average annual values of the observed characteristics between women and men was negatively verified, and the differences in the functional strength trends are probably related to the moderate mass gain in women, with a relative stability of this characteristic in men.
However, the main aim of this study was to determine the magnitude of changes in the examined somatic features and indicators of motor skills conducive to health over a period of two decades of observation.For the observed somatic features, secular trends of the decreased age in men and increased body mass in women, as well as the stability of the body height in both sexes, were found in the student groups 18 .In terms of the body height and mass of female students, the results obtained do not clearly correspond to those of secular trends in other student groups, especially those studied in earlier decades.Previously, secular trends of increasing body height and mass, accompanied by a weaker body structure, were found 15,31,32 .Some of the studies showed a secular trend of an increasing BMI among Polish students, which contradicts the trends for this indicator in the currently analysed groups of female students in the 2011-2020 decade 18,32 .Among the currently examined characteristics, there were also trends towards a decrease in the thickness of the skinfold over the biceps and of that measured over the calf among men, which may indicate a greater activity involving the limbs, especially the upper limbs, than that involving the other parts of the body.It is also possible that there are tendencies for central distribution of adipose tissue and its reduction in the distal parts, which is beneficial in the case of the upper limbs 33,34 .There is scientific evidence of a secular trend of an increasing thickness of skinfolds and their stabilization on the upper torso, as well as increasing obesity among academic youth 13,31 .The reduction in the thickness of skinfolds on the lower leg among the currently studied students should be considered an unfavourable change, especially in light of the significant increase in their waist circumference during the pandemic 18 .This may increase the future risk of cardiovascular disease, especially when combined with the central fat allocation.It can also be a component of insulin resistance, leading to diabetes, hyperlipidaemia and hypertension 35 .However, this statement is of relative value because it is based on the observations of the skinfold thickness and waist circumference and not on detailed measurements of body components.
An increase in women's body mass, with a significant reduction in the thickness of skinfolds at the front of the arm and a trace of this tendency in most other folds, without body height changes, may indicate the muscle mass growth in female students or a general tendency for a slimmer figure and a reduced percentage of obesity in many communities, especially in young ones [36][37][38][39][40] .We consider the above changes to be beneficial, which was probably reflected in the stability of the vast majority of the examined indicators of motor skills.This observation contradicts scientific evidence showing an increase in the thickness of skinfolds or an increase in obesity presented in other studies 15,49 .
It is disturbing, however, that the level of body flexibility in women significantly deteriorated, although women are more predisposed to its manifestation than men are.This may indicate a lack of care by women for this feature, which, without targeted exercise, deteriorates with age.It is therefore possible that in subsequent professional work, women will be more exposed to overloads and injuries of the lower part of the spine, which are becoming common phenomena among physiotherapists 28 .We base this assumption both on general knowledge concerning the interpretation of the results of flexibility tests and on reports on the occurrence of pain in the lumbosacral spine among physiotherapy students 41 .It is also possible that the deterioration of women's flexibility, in addition to the effects of the pandemic, affected the unfavourable trunk strength results obtained in 2020.However, this assumption requires continued research taking into account the effects of the pandemic and the history of SARS-CoV-2 infection.
However, the presented negative secular trends were smaller than expected, especially in the context of numerous reports of deteriorating physical fitness and health status in many societies 6,7,14,32 .Thus, the hypothesis about the deterioration of the components of physical fitness that support the health of young adults over time was mostly negatively verified.Nevertheless, it should still be taken into account that apart from statistically significant secular trend changes over time, there also were trends of deterioration in the levels of static strength of the right hand and endurance in both sexes, as well as in the strength of the abdominal muscles and functional strength in women, which may have intensified over time.We base this supposition on the results of many studies that showed favourable changes in physical development and simultaneous deterioration of physical fitness in young generations [42][43][44][45] .Therefore, special attention should be given to the improvement of the examined components of physical fitness, especially the grip strength, which is an important indicator of energy abilities and can be used in predicting health and well-being, especially in older people 46 .In this study, the results obtained among the studied groups of students did not correspond to the scientific literature data 47,48 .With clearly different levels of static strength between the examined and compared groups, the information about www.nature.com/scientificreports/ the significantly lower strength of the dominant hand was not confirmed either.The vast majority of the people surveyed by us were right-handed, and in all age groups of both men and women, the grip strength of the right hand was slightly greater than that of the left hand.Therefore, we believe that differences in the procedures of the performed tests and the significantly larger number of groups examined in this study than in the other studies contributed to the observed difference.It was not possible to unambiguously determine the predictors of the observed characteristics.Only male sex, body mass and the thickness of suprailiac skinfolds were found to be predictors of several examined motor characteristics, with no influence of sociometric determinants, including the inclusion of the fitness exam in the university recruitment procedure, although two of the three fitness tests, handgrip and endurance shuttle run, were within the scope of the present study.Thus, our results are somewhat surprising and allow us to claim that the too low criteria of the fitness exam were not sufficiently selective for physically fit people, while intellectual selection for studies was the decisive factor and was thus somewhat discriminatory against candidates from lower social class levels.Hence, there was little social diversity among the studied students, and therefore, there were no sociometric features among the predictors of physical fitness tests supporting health.
The presented research results represent only a fragment of the secular trends in the physical fitness of the surveyed physiotherapy students.In addition to the currently studied components, physical fitness includes other components supporting motor achievements, certain body structure regularities, and daily physical activity and skills 20 .In the profession of physiotherapists, these components are of particular value because they allow the use of various types of physical activity in the broadly understood therapeutic process.Thus, the continuation of research allowing for recognition of the area of physical activity and motor skills as well as students' readiness to later use them in their professional work seems fully justified.Issues worth considering in subsequent studies also include the further effects of the pandemic and social changes caused by warfare in Europe.
During the implementation of the present research, certain limitations were encountered, which make the results only relatively relevant.These include the fact that the research covered students from only one medical university and those studying at different levels of education and years of study, as well as the impossibility of conducting research during the lockdown period.Covering first-year students in the study may have obscured the actual results because of the need for young people to adapt to a new reality, which often causes heavy burdens and negative dietary changes, as well as a reduction in recreational and sports activities 49,50 .The small number of men in the groups could also have influenced the observed secular trend of the reducing age of the students.However, we believe that the advantage of this work is the relatively long duration of research carried out with a recognized physical fitness test by one research team, as well as the ability to obtain complete results from a large percentage of students under observation.It was also important to undertake and continue a study, starting from the first recruitment of physiotherapy students at the Polish Medical Academy.Therefore, we believe that the obtained results are relatively reliable and may be helpful for updating physical education programs in physiotherapy studies and for recruiting future candidates.The results may also provide valuable information material for people intending to study physiotherapy.

Conclusions
The results of many years of observation of trends in secular components of physical fitness supporting the health of people studying physiotherapy at the Medical University of Bydgoszcz showed a trace of a negative tendency in changes in all examined motor characteristics.The exceptions were several identified somatic features and the deterioration of flexibility in women.Therefore, attention to the appropriate level of the observed features seems to be justified, and the results of the present research may be helpful in updating physical education programs in physiotherapy studies or in selection of candidates for such studies based on their motor skills.It also seems advisable to continue the research topic, extending it to the area of motor skills, which are useful both in undertaking targeted physical activity and in creating kinesiotherapy exercises conducted with patients.

Figure 2 .
Figure 2. Graphical characterization of secular trends in body mass.

Figure 3 .
Figure 3. Graphical characterization of secular trends in the biceps skinfold.

Table 1 .
Demographic characteristics of the participants.n (%), statistical characteristic and percentage values of the size of the surveyed student groups; mean, average age of the student groups.SD standard deviation.

Table 2 .
Skinfold thickness of the examined men (mean ± SD).SD standard deviation.

Table 3 .
Skinfold thickness of the examined women (mean ± SD).SD standard deviation.

Table 4 .
Statistical characteristics of the men in the fitness tests (mean ± SD).SD standard deviation.

Table 5 .
Statistical characteristics of the women in the fitness tests (mean ± SD).SD standard deviation.

Table 6 .
Relationships of changes in average annual values of the study characteristics between females and males, as determined by Pearson's linear correlation coefficients.Statistically significant correlations are indicated in bold (p < 0.05).

Table 7 .
Coefficients of linear regression models of the form feature = b time + const for male characteristics.Time coef-Coefficient for the time variable (b); time p value-p value for the time variable; const -value of the constant; const p value-p value for the constant; R-square-coefficient of determination.Statistically significant models are marked with bold (p < 0.05).

Table 8 .
Coefficients of linear regression models of the form feature = b time + const for female characteristics.Time coef-Coefficient for the time variable (b); time p value-p value for the time variable; const -value of the constant; const p value-p value for the constant; R-square-coefficient of determination.Statistically significant models are marked with bold (p < 0.05).